This invention relates to video tape recorders (VTR's) and, more particularly, to the generation of a quasi-vertical synchronizing signal when a VTR is operated in its reproducing mode, which quasi-vertical synchronizing signal is timed to occur at a predetermined relative location in the scanning trace of the VTR reproducing head, regardless of the effective length of that trace which may vary from a normal length in accordance with a selected special video effect.
In a helical scan video recording system, such as the so-called omega-type VTR, wherein a single head is used to scan parallel successive record tracks across a magnetic tape which is deployed about a guide drum in a shape resembling the Greek letter .OMEGA., or in a two-head VTR, the scanning trace of the head during a normal recording operation is at a slant, or angle, with respect to the longitudinal direction of the moving tape. Because of this configuration, the parallel tracks which are recorded on the tape are skewed with respect to the direction of tape motion. In general, each track contains a field of video signals and, thus, each track contains successive horizontal line intervals which are divided, broadly, into the video signal information portion and the vertical blanking interval of the field of video signals. In the NTSC system, each track, that is, each recorded field of video signals, contains 262.5 horizontal line intervals.
During a normal reproducing operation, the scanning trace of the reproducing head is in substantial alignment with each recorded track. Recently, a so-called bi-morph assembly, which contains a piezo-ceramic leaf assembly which is responsive to control voltages applied thereto to deflect, or bend, has been provided as a support element for the head. In the event that, during the normal reproducing operation, the scanning trace varies from the recorded track, the bi-morph assembly is controlled so as to deflect in a manner which brings the head into substantial coincidence with the recorded track. This dynamic adjustment of the head during a normal reproducing operation can be turned to account so that the head can be aligned with selected tracks during special video effects operations, such as stop motion, fast forward motion and reverse motion.
When using a VTR for a signal reproducing operation, particularly of the broadcast quality type, the reproduced video signal generally is supplied through a time base corrector in order to correct for various time base errors which may be derived from the VTR, these errors being of the type which introduce undesired jitter or color distortion into the video picture which ultimately is reproduced. A typical time base corrector stores a number of reproduced line intervals, and then reads out these stored line intervals at a standard read-out frequency. In order to insure that the read out video signals from the time base corrector are in the proper vertical phase, that is, the first or top line is read out in proper vertical phase synchronism so as to appear at the top of the video picture, the read-out operation of the time base corrector generally is synchronized with the vertical synchronizing signal which is reproduced by the VTR. If the vertical synchronizing signal of the original video signal is recorded during the end portion of each scanning trace, then, during the normal reproducing operation, the time base corrector is synchronized with this vertical synchronizing signal to read out the first stored line of video information at some predetermined time following its reproduction. For example, if the recorded vertical synchronizing signal precedes the first line of video information by about twelve horizontal line intervals, the time base corrector is synchronized to read out this first line of video information at a delayed time equal to twelve horizontal line intervals following the reproduction of the vertical synchronizing signal.
While this control over the time base corrector generally is satisfactory for a normal reproducing operation, it suffers from some deficiencies when the VTR is operated in a special video effects mode, such as stop motion, fast motion or reverse motion. During such a special video effects mode of operation, the tape speed is changed from its normal speed. For example, for stop motion the tape is stopped so as to permit the head to scan the same recorded track a number of times. For fast motion, the tape is moved in its forward direction at a faster-than-normal speed so that the head scans only alternate tracks (for 2.times. motion) or every third track (for 3.times. motion), or the like. For reverse motion, the tape is moved in the reverse direction, and each recorded track is scanned. During such special video effects, the effective length of the scanning trace of the head as it scans a particular track differs from its normal length. For example, for stop motion, the effective length of the scanning trace along the one recorded track which is scanned thereby is longer than its normal effective length and, therefore, is longer than the length of the scanned track. Similarly, if the VTR is operated in a reverse mode of reproduction, the effective length of the trace which is scanned by the head is still larger than its normal length and, therefore, is larger than the length of the track which it scans. Conversely, when the VTR is operated in its fast motion reproducing mode, the effective length of the scanning trace of the head now is less than its normal length and, consequently, this scanning trace is less than the length of the track which the head is scanning. Because of this varying effective length of the scanning trace of the head, the head will not reach the first, or top line of recorded video information at a time which is delayed by, for example, twelve horizontal line intervals from the reproduction of the vertical synchronizing signal. For longer-than-normal scanning traces, such as during stop motion or reverse motion operations, the head does not reach the first line of recorded video information until about 14.5 or 17 (or more) line intervals following the reproduction of the vertical synchronizing signal. This means that if the reproduced vertical synchronizing signal is used as a reference for reading out the first line of video information from the time base corrector, there will be a resultant vertical displacement in the video picture during these special video effects. For those special video effects in which the effective length of the scanning trace is greater-than-normal, such as during stop motion or reverse motion, the displayed video picture will appear to be shifted in the downward direction; and for those special video effects wherein the scanning trace is greater-than-normal, such as during fast motion, the video picture will appear to be shifted in the upward direction. Furthermore, during continued operation in the special video effects mode, the video picture will appear to vibrate in the vertical direction.
The aforedescribed problem is present either for those VTR systems in which the vertical synchronizing signal actually is recorded or in which only the vertical blanking interval is recorded. In the latter operation, although the vertical synchronizing signal per se is not reproduced, the vertical synchronizing signal is synthesized by counting the number of horizontal line intervals which the head scans, and then supplying a vertical phase synchronizing signal to the time base corrector after a predetermined number of such horizontal line intervals have been counted. However, if the effective length of the scanning trace is greater-than-normal, for example, if this length is equal to about 265 line intervals rather than the normal 262.5 line intervals, the vertical phase synchronizing signal supplied to the time base corrector will be premature. This will result in the same vertical displacement, or vertical vibration in the video picture, as described previously.